The present invention relates to a current zero crossing detecting circuit, particularly to a driving half bridge circuit of Pulse Width Modulation (PWM) type for a brushless motor.
A brushless motor is a synchronous motor the rotation of which is obtained by means of a switching of the currents in the windings synchronously with the rotor position. It is necessary, therefore, knowing exactly the rotor position so as to obtain the best precision for the motor working and this position is usually deduced by means of two approaches: 1) by means of Hall effect position sensors; 2) by means of a counter electromotive force signal (BEMF).
In this last embodiment the driving current curve must coincide with the curve of BEMF to obtain the maximum efficiency of driving.
In the driving of brushless motors, therefore, the current zero crossing in the motor windings has to be identified so as to realize a synchronization with the BEMF zero crossing.
Presently, in order to establish the current zero crossing in the PWM driving half bridge circuits a resistance, disposed in series to a single motor winding, connected with a specific comparator, so as to detect the voltage zero crossing, as shown in FIG. 1, is used.
The drawback of this embodiment resides in the use of an external component, in the specific case a resistance, for reading the voltage value.
In view of the state of the art described, it is an object of the present invention detecting the current zero crossing information without the necessity of the presence of an external component.
According to the present invention, such object is achieved by a current zero crossing detecting circuit including a PWM driving half bridge circuit, which generates an output signal and a signal synchronous with the high impedance condition of said PWM driving half bridge circuit, characterized by comprising detecting means synchronous with said signal synchronous with the high impedance condition of said PWM driving half bridge circuit and said output signal, and said detecting means generating a direction signal showing the current direction flowing in said pulse width modulation circuit.
Thanks to the present invention it is possible making a circuit adapted to detect the current zero crossing without the help of any devices external to the PWM driving circuit.
Moreover, thanks to the present invention it is also possible obtaining a detecting circuit of easier realization.